This invention relates generally to the conversion of hydrocarbons and, more specifically, to contact material compositions and oxidative conversion processes using such compositions.
As the uncertain nature of the limited supplies of and access to crude oil has become increasingly apparent, alternative sources of hydrocarbons and fuels have been sought out and explored. The conversion of low molecular weight alkanes (lower alkanes) to higher molecular weight hydrocarbons has received increasing consideration as such low molecular weight alkanes may be generally available from more readily secured and reliable sources. Natural gas, partially as a result of its comparative abundance, has received a large measure of the attention that has focused on sources of low molecular weight alkanes. Large deposites of natural gas, mainly composed of methane, are found in many locations throughout the world. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during numerous mining operations, in various petroleum processes, and in the above- or below-ground gasification or liquefaction of coal, tar sands, oil shale and biomass, for example.
Today, much of the readily accessible natural gas generally has a high valued use as a fuel whether in residential, commercial or in industrial applications. Additional natural gas resources, however, are prevalent in many remote regions of the world, such as remote areas of Western Canada, Africa, Australia, U.S.S.R. and Asia. Commonly, natural gas from these remote resources is referred to as "remote natural gas" or, more briefly, "remote gas."
In many such remote regions, the widespread, direct use of the natural gas as a fuel is generally not currently profitable. Further, the relative inaccessibility of gas from such resources is a major obstacle to the more effective and extensive use of remote gas as the transportation of the gas to distant markets wherein the natural gas could find direct use as a fuel is typically economically unattractive.
Of course, while the primary current use of natural gas is as a fuel, natural gas may alternatively be used as a feedstock for chemical manufacture. In fact, natural gas is a primary chemical feedstock for the manufacture of numerous chemicals, such as methanol, ammonia, acetic acid, acetic anhydride, formic acid, and formaldehyde, for example. However, the markets for such chemicals are fairly limited in size. Consequently, methods for converting low molecular weight alkanes, such as those present in remote natural gas, to higher molecular weight hydrocarbons, preferably, to more easily transportable liquid fuels for which the world market is relatively large and/or elastic, are desired and a number of such methods have been proposed or reported.
Conversion of natural gas to liquid products is a promising solution to the problem of more effectively and efficiently utilizing low molecular weight hydrocarbons from remote areas and constitutes a special challenge to the petrochemical and energy industries. The dominant technology currently employed for the utilization of remote natural gas involves conversion of the natural gas to a liquid form via the formation of synthesis gas, i.e., a process intermediary composed of a mixture of hydrogen and carbon monoxide also commonly referred to as "syngas." In syngas processing, methane, the predominant component of natural gas, although typically difficult to activate, is reacted with oxygen or oxygen-containing compounds such as water or carbon dioxide to produce syngas which in turn is then converted to desired products.
Syngas processing, however, is relatively costly as the production of syngas and the subsequent conversion of the syngas are typically very capital intensive processing schemes. Further, while some of the products to which syngas can be converted, such as methanol, mixed alcohols, acetic acid, etc., contain oxygen and are thus logical products for production via syngas processing, hydrocarbon products such as gasoline and diesel fuel typically do not contain oxygen and consequently the production of such materials via syngas processing requires the additional processing step of oxygen removal. Consequently, when such products are produced via syngas processing, the addition and later removal of oxygen ultimately increases the cost of production.
When hydrocarbon products such as gasoline and diesel fuel are sought, the syngas mixture can be converted to syncrude, such as with Fischer-Tropsch technology, and then upgraded to the desired transportation fuels using typical refining methods. Alternatively, syngas can be converted to liquid oxygenates which can be blended with conventional transportation fuels to form materials such as gasohol, used as an alternative fuel or converted to conventional transportation fuels by catalysts such as certain zeolites.
Because syngas processing typically requires high capital investment, with syngas typically being produced in energy intensive ways such as by steam reforming where fuel is burned to supply the heat of reforming, and represents an indirect means of higher hydrocarbon production (i.e., such processing involves the formation and subsequent reaction of the syngas intermediaries), other means for converting lower alkanes directly to higher hydrocarbons have been sought.
Oxidative coupling has been recognized as a promising approach to the problem of conversion of lower alkanes to higher molecular weight hydrocarbons. The mechanism of action of oxidative coupling processing, however, has not been clearly identified or defined and is not clearly understood. In such oxidative coupling processing, a low molecular weight alkane or a mixture containing low molecular weight alkanes, such as methane, is contacted with a solid material referred to by various terms including catalyst, promoter, oxidative synthesizing agent, activator or contact material. In such processing, the methane is contacted with such a "contact material" and, depending on the composition of the contact material, in the presence or absence of free oxygen gas, is directly converted to ethane, ethylene, higher hydrocarbons and water. Carbon dioxide, the formation of which is highly favored thermodynamically, is an undesired product, however, as the formation of carbon dioxide results in both oxygen and carbon being consumed without production of the desired higher value C.sub.2+ hydrocarbons.
Catalytic mixtures containing reducible metal oxides are highly active and many are 100% selective for producing CO.sub.2, that is, they are combustion catalysts. In order to obtain desired selectivity for hydrocarbon formation, Group IA metals, particularly lithium and sodium, have been used in such catalytic mixtures. Under the conditions used for oxidative coupling, however, migration and loss of the alkali metal normally occurs. In order to avoid complete combustion most methods for oxidative conversion have been carried out in the absence of an oxygen-containing gas, relying on the oxygen theoretically being supplied by the catalyst.
Nevertheless, in most cases involving oxidative coupling processing of methane, carbon monoxide and hydrogen are coproduced in addition to desired C.sub.2+ hydrocarbons. If desired, such coproduced hydrogen can be used alone, in part or in its entirety, or supplemented with hydrogen from another source to effect conversion of carbon oxides to produce methane. Such produced methane can, in turn, be recycled for desired oxidative coupling processing. Alternatively, the hydrogen can be used to effect conversion of carbon monoxide to carbon-containing oxygenates such as methanol or mixed alcohols (e.g., a mixture of one or more alcohols such as methanol, ethanol, propanols and butanols) or higher hydrocarbons such as a mixture of paraffins and olefins typically produced in the process commonly known as Fischer-Tropsch synthesis. Alternatively or in addition, such coproduced carbon monoxide and hydrogen can, if desired, be combined with olefins, such as those produced during the oxidative coupling processing, to produce various oxygenates, such as propanol, for example. As described above, however, the production of materials such as oxygenates from carbon monoxide and hydrogen (i.e., synthesis gas) is not a direct approach for the utilization of natural gas, as such processing still involves the use of the syngas intermediaries.
Furthermore, the processing of coproduced hydrogen and carbon monoxide typically increases the cost of any such processing scheme. Thus, the need for active oxidative coupling contact materials which have relatively high selectivities for desired higher hydrocarbons and which contact materials are stable and have long life (i.e., maintain relatively high levels of activity and selectivity to higher hydrocarbons over extended periods of use without the need for regeneration or replacement).
Many patents describe processes for converting methane to heavier hydrocarbons in the presence of reducible metal oxide catalysts. During such processing, the reducible metal oxide "catalyst" typically is reduced and thus most of these patents require or imply the need for a separate stage to reoxidize the catalyst.
For example, U.S. Pat. No. 4,444,984 discloses a method for synthesizing hydrocarbons wherein methane is contacted with a reducible oxide of tin at an elevated temperature. Such contact results in the tin oxide being reduced. The reduced composition is then oxidized with molecular oxygen to regenerate a reducible oxide of tin.
U.S. Pat. No. 4,495,374 discloses the use of a reducible metal oxide promoted by an alkaline earth metal in such a method of methane conversion. During such processing, the reducible metal oxide of the promoted oxidative synthesizing agent is reduced. The reduced synthesizing agent can then be removed to a separate zone wherein it is contacted with an oxygen-containing gas to regenerate the promoted oxidative synthesizing agent.
Examples of other such patents include: U.S. Pat. No. 4,523,049, which shows a reducible oxide catalyst promoted by an alkali or alkaline earth metal, and requires the presence of oxygen during the oxidative coupling reaction; U.S. Pat. No. 4,656,155, which specifies a reducible metal oxide in combination with an oxide of zirconium, an oxide of yttrium and, optionally, an alkali metal; U.S. Pat. No. 4,450,310, which is directed to coupling promoted by alkaline earth metal oxides in the total absence of molecular oxygen; and U.S. Pat. No. 4,482,644, which teaches a barium-containing oxygen-deficient catalyst with a perovskite structure.
Several patents describe catalyst for higher hydrocarbon synthesis which can include a Group IIA; a metal of scandium, yttrium or lanthanum; and/or other metal oxides.
Commonly assigned U.S. Pat. No. 4,931,311 discloses a catalyst composition comprising a mixed oxide of:
a) a Group IIIB metal selected from the group consisting of yttrium, scandium and lanthanum;
b) a Group IIA metal selected from the group consisting of barium, calcium and strontium; and
c) a Group IVA metal selected from the group consisting of tin, lead and germanium, with the Group IIIB, Group IIA and Group IVA metals in an approximate mole ratio of 1:0.5-3:2-4, respectively.
U.S. Pat. No. 4,780,449 discloses a catalyst including metal oxides of a Group IIA metal, a Group IIIA metal, a lanthanide series metal excluding Ce, or mixtures thereof. The patent lists as optional promoter materials metal oxides of a metal of Groups IA, IIA, IIIA, IVB, VB, IB, the lanthanide series, or mixtures thereof.
Catalysts which contain metal oxides which are reduced under the reaction conditions of use are typically physically and/or chemically relatively unstable under the reaction conditions of use. That is, such catalysts generally do not maintain needed or desired physical and/or chemical characteristics for extended periods of time (e.g., such characteristics as reactivity and physical form are typically not maintained for more than a few minutes) without regeneration, reformation or other remedial procedures.
Also, as the reducible metal oxides of such materials typically undergo chemical reduction with use, the activity of the materials for producing desired products, such as C.sub.2+ hydrocarbons in the oxidative coupling processing of methane, for example, worsen.
For example, with contact materials containing reducible metal oxides, the problem of over-reduction is typically associated with the reduction of the metal oxide to the metal. Often, the selectivity of the contact material changes dramatically when the material has been over-reduced, leading to a material which results in combustion reactions or which results in the formation of mixtures of carbon oxides with water and hydrogen when the material is used in the oxidative coupling of lower alkanes such as methane, for example. Some reducible metal oxide-containing contact materials (e.g., manganese oxide) at temperatures in the range of about 800.degree. to about 1,000.degree. C. and in the absence of oxygen, once over-reduced are very difficult to reoxidize and a permanent or near permanent alteration in the characteristics of the material occurs. In some cases, the reduced metal can react with other materials in the composition to form a new phase which is difficult to reoxidize and the contact material is permanently damaged by over-reduction. Such alterations can, for example, result in a loss in selectivity to C.sub.2+ hydrocarbons when the material is used in the oxidative coupling of methane.
Furthermore, contact materials containing metal oxides which are reducible under the reaction conditions of use can, during such use, experience physical deterioration, e.g., breaking apart. Such physical deterioration results, at least in part, from changes in the material during oxidation and reduction. Frequently, the material in its various oxidation states has very different densities, e.g., the material contracts and swells as it is reduced and oxidized. The smaller particles or powders, frequently referred to as "fines," resulting when the material undergoes physical degradation results in pressure drop buildups (in fixed bed operation) and leads to loss of contact material (in fluid bed operation).
In fluid bed operation, fines are frequently carried out with the vapors from the reactor. Additionally, the fines are generally not easily separated from the product gases in common separating devices such as cyclones. Thus, costly separation techniques are required to effect separation of the fines from the product gases. The loss of contact material in the form of fines also necessitates the addition of more contact material to the process to replace that which has been lost and thereby increases the cost of such processing.
The search for a stable, long-lived contact material having high activity and selectivity in the oxidative conversion processing of hydrocarbons has continued.